In construction, a firewall is a fireproof or fire-resistant wall designed to prevent the spread of fire through a building or a vehicle. When the term is applied to a computer system, it refers to a security system consisting of hardware, software or a combination thereof, that limits the exposure of a computer or computer network to attack from unauthorized users. The firewall operates within a networked environment to prevent communications forbidden by a security policy, (typically designed by a system administrator). The firewall attempts to control traffic between different zones of trust. One situation in which a firewall is commonly used is on local area networks that are connected to the Internet to control traffic between the Internet (not trusted) and the internal network (highly trusted). Thus the goal of the firewall is to provide controlled connectivity between zones of differing trust levels by enforcement of a security policy. The proper configuration of firewalls demands skill and a good understanding of network protocols and computer security. Small mistakes can render a firewall worthless as a security tool.